When Does Grass Begin to Grow in The New Year?
It all depends on how warm the soil is. Soil temperature determines whether soil microorganisms, such as bacteria and fungi, are active or not. These organisms transform the raw materials of the soil into available nutrients for plant roots. No grass will grow without active soil organisms to help them.
Soil temperature mirrors air temperature. So instead of putting a thermometer in the ground, you can use a formula called “T-Sum 200” to track soil temperatures indirectly and determine when grass will begin to grow.
Here’s How it Works
Each day, starting January 1, write down that day’s high and low temperature. Plug those numbers into the formula below to calculate the day’s “soil heat degrees.” A day has positive heat degrees if the daily temperature average is above freezing (32 degrees F), or zero heat degrees if it is below freezing. There are no negative heat degree-days no matter how cold it gets.
Note that T-Sum 200 only indicates when existing grass will start growing. The soil is still not warm enough for seeds to germinate. Wait until late March or early April to apply grass seed.
Note, too, that at T-Sum 200, the grass has a long growing season ahead of it. It isn’t growing rapidly yet in February, so it can’t absorb lots of nutrients from manure or fertilizer. However, a light application of compost or manure is okay. Nor is this grass ready to be grazed or trampled. The ground is still saturated, and very susceptible to compaction. (Compaction squeezes air spaces out of the soil that are needed for root growth and water permeation.) Wait until the grass is six to eight inches tall before modestly grazing and then only if the ground isn’t too saturated.
The formula for calculating each day’s heat degrees is:
(the day’s high temperature + low temperature)/2 = average daily temperature (degrees F)
(average temperature – 32 degrees F) x 5/9 = heat degrees for the day
(47 degrees – 34 degrees)/2 = 40.5 degrees average temperature
(40.5 degrees – 32 degrees) x 5/9 = 4.7 heat degrees
Add each day’s heat degrees together, starting with January 1 (January 1 + January 2 + January 3… etc.) When the total heat degrees reaches 200, soil is warm enough for grass to grow. This usually occurs in February.
When Does T-Sum Typically Occur?
T-Sum 200 usually occurs around February 8 in the Everett, Washington area. Since 2001, it has occurred as early as February 4 and as late as February 23. T-Sum 200 for the Arlington area usually occurs a few days later than Everett. Temperature data for other local cities has not been readily available.
If you’d like to track T-Sum 200 for your immediate area, you’ll need a source of information for that day’s high and low temperatures. It’s easy to get a weather forecast for a specific area but it can be hard to find information for a previous day’s weather, although your local newspaper may carry the previous day’s temperatures and precipitation in their archive.
Another, more fun option for tracking daily high and low temperatures is to set up your own weather station. You can purchase relatively inexpensive yet accurate weather stations to track temperatures at your farm or house. They are easy to set up and provide a wealth of useful weather related information.
Tracking T-Sum 200 is a good habit to develop as you start your new grass year. It’s a way of actively thinking about the grass life cycle from the first day. One of the most important skills in grass management is observation, and T-Sum 200 is a tool that heightens your observation skills of weather and plant life.
When T-Sum is Achieved, What Next?
When T-Sum 200 is close, you may want to do an inexpensive soil test to determine your fertilizer needs. This is also a good time to observe the population of weeds in your pasture because they’ll be waking up, too. If your pasture is small enough and weeds are not overwhelming, you may be able to pull them up by hand. It’s a bit early to use herbicides because rain will wash off the chemicals. In most cases, (blackberries being one of them), plants need to be growing very actively for chemicals to be effective. A better option would be to encourage optimum grass growth so that the grass out-competes the weeds. You can spread a light layer of compost after T-Sum 200 has been reached — if you can do it without causing too much soil compaction with your equipment.